Increased incidence of EBV-related disease following paediatric stem cell transplantation with reduced-intensity conditioning

The incidence of Epstein-Barr virus (EBV) viraemia and lymphoproliferative disease (LPD) was studied in a consecutive cohort of 128 paediatric patients undergoing stem cell transplantation (SCT) with reduced-intensity conditioning (RIC; n = 65) or conventional-intensity conditioning (CIC; n = 68). Following CIC, six of 68 (8%) developed viraemia; all remained asymptomatic. EBV viraemia (23 of 65 patients = 35%, P < 0.001) and LPD (10 of 65 = 15%, P < 0.001) were significantly more frequent following RIC. Of the 23 RIC patients who developed viraemia, eight remained asymptomatic, five had symptomatic viraemia (fever +/- rash), and 10 patients developed LPD, two of whom died. An absolute lymphocyte count of <0.3 x 10(9)/l at the time of onset of viraemia was strongly predictive of development of LPD (P < 0.05) in this group. The incidence of viraemia was significantly higher in patients receiving serotherapy with antithymocyte globulin (ATG; 15 of 43, 35%) than Campath (12 of 73, 16.4%, P < 0.05). Primary immunodeficiency and acute graft-versus-host disease were associated with EBV viraemia in univariate analysis, but were not independent risk factors. In conclusion, EBV viraemia and LPD appear to be significantly more common in children following RIC SCT, particularly with selective depletion of recipient T cells relative to B cells following the use of ATG. This probably reflects the profound immunosuppression following RIC SCT, together with the incomplete ablation of recipient-derived B cells.

The impact of telomere erosion on memory CD8+ T cells in patients with X-linked lymphoproliferative syndrome

Patients with X-linked lymphoproliferative syndrome (XLP) experience excessive T cell proliferation after primary Epstein-Barr virus (EBV) infection, due to mutations in the signalling lymphocyte activation molecule (SLAM) associated protein (SAP) molecule. We examined the impact of dysfunctional proliferative control on the extent of CD8+ T cell differentiation in XLP patients who recovered from primary EBV infection. Although these young patients have normal numbers of lytic and latent EBV-epitope-specific CD8+ T cells, they were extremely differentiated as defined by loss of CCR7 and CD27, low telomerase activity and very short telomeres. This was not a direct effect arising from the loss of SAP, but was due to excessive T cell stimulation due to this defect. Thus, transduction of XLP CD8+ T cells with the catalytic component of telomerase (hTERT), but not SAP, prevented telomere loss and considerably extended proliferative lifespan in vitro. These results indicate that excessive proliferation in CD8+ T cells in XLP patients may lead to end-stage differentiation and loss of functional EBV-specific CD8+ T cells through replicative senescence. This may contribute to the defective immunity found in XLP patients who survive acute EBV infection who develop EBV-related B cell lymphomas before the fourth decade of life.

T Cell Suicide Gene Therapy to Aid Haematopoietic Stem Cell Transplantation

Graft versus host disease (GVHD) is a T cell mediated phenomenon that arises following allogeneic haematopoietic stem cell transplantation, and may be particularly severe in the context of human leukocyte antigen (HLA) mismatched procedures. Although GVHD can be largely abrogated through T cell depletion, such measures result in loss of graft potency and reduced anti-viral and anti-leukaemic effects. The genetic modification of T cells to carry a suicide gene mechanism has been advocated as means of allowing T cells to be harnessed for their beneficial effects, and safely eliminated in the event of significant GVHD. The feasibility of the strategy has been demonstrated in clinical studies using T cells modified by retroviral transduction to encode the herpes simplex thymidine kinase (HSVTK) gene to treat patients with haematological malignancies. However, a number of limitations associated with current protocols have become apparent. Most notably, the process of retroviral transduction, which requires pre-activation of T cells, appears to impair subsequent functional potential. Efforts are now directed towards circumventing the pre-activation requirements of retroviral vectors by using alternative lentiviral systems, in association with improved suicide gene/prodrug combinations.

Improved survival after unrelated donor bone marrow transplantation in children with primary immunodeficiency using a reduced-intensity conditioning regimen

The optimal approach to stem cell transplantation in children with immunodeficiency who lack a matched family donor is controversial. Unrelated donor stem cell transplantation gives equivalent outcome to mismatched family donor stem cell transplantation in severe combined immunodeficiency, whereas unrelated donors may be preferable in non–severe combined immunodeficiency children. However, unrelated donor stem cell transplantation with conventional conditioning regimens has been associated with significant treatment-related toxicity, particularly in non–severe combined immunodeficiency patients with preexisting organ dysfunction. We report the outcome of a series of 33 consecutive unrelated donor transplantations performed at our center in children with primary immunodeficiency using a reduced-intensity conditioning regimen between 1998 and 2001. We have compared these outcomes with a retrospective control cohort of 19 patients who underwent transplantation with myeloablative conditioning between 1994 and 1998. All children in both groups had primary engraftment. There was no statistical difference in the speed of immune reconstitution or incidence of graft-versus-host disease between the 2 groups. Overall survival was significantly better in the reduced-intensity conditioning group: 31 (94%) of 33 patients survived, compared with 10 (53%) of 19 in the myeloablative conditioning group (P = .014). We conclude that the reduced-intensity conditioning regimen results in improved survival and reduced transplantation-related mortality compared with myeloablative conditioning in high-risk patients undergoing unrelated donor transplantation.

Gene therapy of X-linked severe combined immunodeficiency by use of a pseudotyped gammaretroviral vector

BACKGROUND

X-linked severe combined immunodeficiency (SCID-X1) is caused by mutations in the common cytokine-receptor gamma chain (gamma(c)), resulting in disruption of development of T lymphocytes and natural-killer cells. B-lymphocyte function is also intrinsically compromised. Allogeneic bone-marrow transplantation is successful if HLA-matched family donors are available, but HLA-mismatched procedures are associated with substantial morbidity and mortality. We investigated the application of somatic gene therapy by use of a gibbon-ape-leukaemia-virus pseudotyped gammaretroviral vector.

METHODS

Four children with SCID-X1 were enrolled. Autologous CD34-positive haemopoietic bone-marrow stem cells were transduced ex vivo and returned to the patients without preceding cytoreductive chemotherapy. The patients were monitored for integration and expression of the gamma(c) vector and for functional immunological recovery.

FINDINGS

All patients have shown substantial improvements in clinical and immunological features, and prophylactic medication could be withdrawn in two. No serious adverse events have been recorded. T cells responded normally to mitogenic and antigenic stimuli, and the T-cell-receptor (TCR) repertoire was highly diverse. Where assessable, humoral immunity, in terms of antibody production, was also restored and associated with increasing rates of somatic mutation in immunoglobulin genes.

INTERPRETATION

Gene therapy for SCID-X1 is a highly effective strategy for restoration of functional cellular and humoral immunity.

Prevalence of SAP gene defects in male patients diagnosed with common variable immunodeficiency

The molecular basis of common variable immunodeficiency (CVID) is undefined, and diagnosis requires exclusion of other diseases including X-linked lymphoproliferative disease (XLP). This rare disorder of immunedysregulation presents typically after Epstein-Barr virus infection and results from defects in the SAP (SLAM associated protein) gene. SAP mutations have been found in a few patients diagnosed previously as CVID, suggesting that XLP may mimic CVID, but no large-scale analysis of CVID patients has been undertaken. We therefore analysed 60 male CVID and hypogammaglobulinaemic patients for abnormalities in SAP protein expression and for mutations in the SAP gene. In this study only one individual, who was found later to have an X-linked family history, was found to have a genomic mutation leading to abnormal SAP cDNA and protein expression. These results demonstrate that SAP defects are rarely observed in CVID patients. We suggest that routine screening of SAP may only be necessary in patients with other suggestive clinical features.

Pathology of astrovirus associated diarrhoea in a paediatric bone marrow transplant recipient

Human astrovirus infection often causes outbreaks of self limiting diarrhoea, but may also infect patients who are immunodeficient or immunocompromised. Although there are previous publications relating to various aspects of astroviruses, there is a minimal amount of literature on the histopathological features of gastrointestinal astrovirus infection in humans. We report the histopathological findings, including immunohistochemical and electron microscopic features, of astrovirus infection in a bone marrow transplant recipient aged 4 years with diarrhoea. The appearance of a small intestinal biopsy did not suggest graft versus host disease, but demonstrated villous blunting, irregularity of surface epithelial cells, and an increase in lamina propria inflammatory cell density. Immunohistochemical staining with a murine astrovirus group specific monoclonal antibody demonstrated progressively more extensive staining in the duodenal and jejunal biopsies, predominantly restricted to the luminal surface and cytoplasm of surface epithelial cells, most marked at the villus tips. Electron microscopic examination demonstrated viral particles within the cytoplasm of enterocytes, focally forming paracrystalline arrays.

Gene therapy for severe combined immune deficiency

Infants born with severe combined immune deficiencies are prone to life-threatening infections and, without treatment, do not survive beyond the first year of life. Haematopoietic stem cell transplantation from a fully matched donor offers the possibility of cure. In the absence of a suitable matched donor, haploidentical transplants from a parental donor may be undertaken, but these are associated with more complications and lower success rates. Recently, an alternative therapeutic option based on retroviral gene delivery has been used to correct X-linked severe combined immune deficiency (SCID-X1) and adenosine deaminase deficiency. Clinical trials have established that in situations where ex vivo gene transfer into haematopoietic progenitor cells confers a strong selective advantage, the procedure is a feasible alternative to haploidentical transplantation, with favourable kinetics of immune reconstitution.

SAP mediates specific cytotoxic T-cell functions in X-linked lymphoproliferative disease

Cytotoxic T cells (CTLs) and natural killer cells play a major role in the immune response to Epstein-Barr virus (EBV) infection. In X-linked lymphoproliferative (XLP) disease, a severe immunodeficiency, immunodysregulatory phenomena are observed following EBV infection, suggesting that defects exist in these effector populations. The gene defective in XLP is SAP (signaling lymphocytic activation molecule [SLAM]–associated protein), an adaptor protein that mediates signals through SLAM and other immunoglobulin superfamily receptors including 2B4. We generated EBV-specific T-cell lines from controls and XLP patients and examined CTL function in response to different stimuli. We show that XLP patients can generate EBV–T-cell lines that are phenotypically similar to those from controls. XLP patient EBV–T-cell lines showed a significant decrease in interferon-gamma (IFN-γ) production in response to 2B4 and autologous EBV-transformed lymphoblastoid cell line (LCL) stimulation but not in response to SLAM. Furthermore, XLP EBV–T-cell lines demonstrated markedly decreased cytotoxic activity against autologous LCLs. By retroviral gene transfer of the SAP gene into XLP EBV–T-cell lines, we show reconstitution of IFN-γ production and of cytotoxic activity confirming SAP-dependent defects. These studies demonstrate that in XLP the lack of SAP affects specific signaling pathways resulting in severe disruption of CTL function.

Gene therapy progress and prospects: gene therapy for severe combined immunodeficiency

Severe combined immunodeficiencies have long been targeted as a group of disorders amenable to gene therapy because of their defined molecular biology and pathophysiology, and the prediction that corrected cells would have profound growth and survival advantage. Recently, several clinical studies have shown that conventional gene transfer technology can produce major beneficial therapeutic effects in these patients, but, as for all cellular and pharmacological treatment approaches, with a finite potential for toxicity.

Non-myeloablative stem cell transplantation for congenital immunodeficiencies

Allogeneic haematopoietic stem cell transplantation (HSCT) can be a highly successful treatment option for individuals with congenital immunodeficiency states. The strategy for HSCT is varied but in cases where there is preservation of residual T cell function, conditioning regimes have been used and have been based around a combination of busulphan and cyclophosphamide with or without serotherapy. In patients with coexisting organ damage this has resulted in significant morbidity and mortality. We have therefore used a low-intensity conditioning regime for transplantation in this group of immunodeficiency patients. Twenty-one patients with a variety of different immunodeficiencies were treated using the following conditioning regimes: (1) fludarabine/melphalan/ATG or Campath 1H (n=16), (2) fludarabine/cyclophosphamide/Campath 1H (n=1), (3) TBI/CyA/MMF (n=1), (4) fludarabine/melphalan/busulphan/ATG (n=3). In 13 cases matched (n=9) and 1 Ag mismatched (n=4) unrelated donors were used and in eight cases transplants from matched siblings (n=4), 1 Ag mismatched sibling (n=1), matched parent (n=1) and haploidentical parents (n=3) were performed. At a median follow-up of 13 months, 19 of 21 (90%) patients were still alive following the transplant procedure. Despite a T cell replete graft and the use of unrelated donor grafts in the majority of patients studied there was no evidence of significant organ disease. Immune reconstitution in terms of CD3+ and CD4+ T cell recovery and function was equivalent in comparison with a historical cohort. We believe that this low-intensity approach has significant implications for transplantation of individuals with immunodeficiency states with established organ disease.

T cell transduction and suicide with an enhanced mutant thymidine kinase

Retroviral transfer of Herpes simplex virus thymidine kinase to T cells has been used to confer sensitivity to the antiviral agent ganciclovir. This has allowed therapeutic approaches to be developed in which T cells mediating graft-versus-host disease after bone marrow transplantation can be selectively eliminated by the administration of ganciclovir. Although the strategy has been shown to be generally successful in early clinical trials, there are concerns about possible resistance to ganciclovir and the risk of myelosuppressive side-effects at the doses required to induce T cell suicide. We have incorporated the enhanced mutant HSV-TKSR39 into retroviral vectors tailored to exhibit high levels of expression in T cells and have used protocols optimized for the transduction and selection of primary lymphocytes. We demonstrate that leukemic and primary T cells can be efficiently transduced and highly enriched under conditions that should be readily adaptable for clinical use. T cells carrying HSV-TKSR39 were inhibited by exposure to ganciclovir at concentrations an order of magnitude below those required for wild-type HSV-TK. The less toxic agent aciclovir also eliminated T cells transduced with HSV-TKSR39 (but not HSV-TK), underlining the increased therapeutic potential of the mutant suicide gene system in the bone marrow transplantation setting.

X-linked lymphoproliferative disease: three atypical cases

Common variable immunodeficiency (CVID) is the most frequently occurring primary immunodeficiency in both children and adults. The molecular basis of CVID has not been defined, and diagnosis involves exclusion of other molecularly defined disorders. X-linked lymphoproliferative disease (XLP) is a rare disorder in which severe immunodysregulatory phenomena typically follow Epstein-Barr virus (EBV) infection. Boys who survive initial EBV infection have a high incidence of severe complications, including progressive immunodeficiency, aplastic anaemia, lymphoproliferative disease and lymphoma. Survival beyond the second decade is unusual, although bone marrow transplantation can be curative. Until recently reliable diagnostic testing for XLP has not been available, but the identification of the XLP gene, known as SH2D1A, and coding for a protein known as SAP, means that molecular diagnosis is now possible, both by protein expression assays, and mutation detection, although the mutation detection rate in several series is only 55-60%. We describe three male patients initially diagnosed as affected by CVID, one of whom developed fatal complications suggestive of XLP, and all of whom lack expression of SAP. Two out of three have disease-causing mutations in the SAP gene, consistent with published data for XLP. These findings raise the possibility that a subgroup of patients with CVID may be phenotypic variants of XLP. Further studies are necessary to investigate this possibility, and also to clarify the prognostic significance of SAP abnormalities in such patients in the absence of typical features of XLP.

Defective expression of the interleukin-2/interleukin-15 receptor beta subunit leads to a natural killer cell-deficient form of severe combined immunodeficiency

Development of T and natural killer (NK) cells is critically dependent on cytokine signaling, and defects in cytokine receptor complex subunits have been shown to result in severe combined immunodeficiency (SCID) syndromes in humans and in murine models. An infant boy had typical clinical features of SCID and was found to lack NK cells in his peripheral circulation. Molecular analysis did not reveal abnormalities in his gammac or JAK-3 genes, and he was investigated for defects in the interleukin-15 (IL-15) receptor complex because functional IL-15 signaling is essential for NK cell development. Expression of the IL-2R/IL-15Rbeta chain was significantly reduced in the patient's peripheral blood mononuclear cells (PBMCs) by immunoblot, flow cytometry, and Northern blot analysis. Furthermore, IL-2 stimulation of PBMCs showed only minimal tyrosine phosphorylation of JAK-3. These data demonstrate that defects in IL-2R/1L-15Rbeta expression can lead to a unique NK-deficient SCID immunophenotype. (Blood. 2001;98:877-879)

Cognitive and behavioral abnormalities in adenosine deaminase deficient severe combined immunodeficiency

OBJECTIVE

The objective was to evaluate the cognitive, behavioral, and neurodevelopmental function in patients with adenosine deaminase deficient severe combined immunodeficiency (ADA-SCID) and to compare the findings with those of a case control group of patients without ADA-SCID.

STUDY DESIGN

Case-matched pairs of patients with ADA-SCID (n = 11) and patients without ADA-SCID who had undergone bone marrow transplantation were recruited. Subjects were assessed by age-appropriate standard tests of intelligence, behavior, and neurodevelopment.

RESULTS

Cognitive ability was not significantly different between the 2 groups, but patients with ADA-SCID showed a significant inverse correlation between deoxyadenosinetrisphosphate levels at diagnosis and IQ (P =.048). Behavioral assessment showed that patients with ADA-SCID functioned in the pathologic range on all domains, whereas mean scores for the control group were within normal limits. Behavioral impairment in patients with ADA-SCID also showed a significant positive correlation with age (P =.026).

CONCLUSIONS

Cognitive function in ADA deficiency is adversely affected by the severity of metabolic derangement at the time of diagnosis. In addition, patients with ADA-SCID have significant behavioral abnormalities after transplantation. These defects are not due to the transplant procedure but reflect the systemic nature of ADA deficiency. These findings have important implications for future medical and nonmedical management strategies.

Protein assays for diagnosis of Wiskott-Aldrich syndrome and X-linked thrombocytopenia

Mutations in the gene encoding the Wiskott-Aldrich syndrome protein (WASp) give rise to Wiskott-Aldrich syndrome (WAS), a condition that exhibits a wide spectrum of clinical severity. Patients may develop mild thrombocytopenia or suffer from a wide range of associated disorders including eczema, immune dysfunction, autoimmune disease and malignancy. The clinical diagnosis of Wiskott-Aldrich syndrome (WAS) can be difficult and is usually supported by the detection of WASp gene mutations using genetic analysis. Recently, protein-based assays have been used to demonstrate the absence of WASp in patients known to have WASp gene mutations. We have now reversed this approach and report on the use of immunoblot assays to rapidly diagnose WAS in 13 patients. There was a complete absence of WASp in 10 out of 13 patients and an abnormal protein form was detected in the remaining three patients. In all cases, subsequent genetic analysis confirmed the presence of a WASp gene mutation. We believe that protein-based assays should be employed as the first line of investigation in the diagnosis of WAS spectrum disorders.

Rapid protein-based assays for the diagnosis of T-B+ severe combined immunodeficiency

The severe combined immunodeficiencies (SCID) are a heterogeneous group of conditions arising from a variety of molecular defects. The X-linked form of SCID (X-SCID) is caused by defects in the common gamma chain (gammac), and is characterized by a T-B+NK- immunophenotype. This lymphocyte profile is seen in an autosomal recessive form of SCID caused by mutations in the JAK3 molecule. Thus, X-SCID and JAK3-deficient SCID are clinically and immunologically indistinguishable. Knowledge of the precise molecular defect is essential for antenatal diagnosis, carrier testing and for treatment using somatic gene therapy. To identify the molecular defect in children presenting with a T-B+NK- form of SCID, we have developed rapid assays based on flow cytometric analysis of gammac, immunoblotting for JAK3 and gammac, and detection of interleukin-2 (IL-2)-induced tyrosine phosphorylation of JAK3. Sixteen T-B+NK- SCID patients from 15 families were examined. Nine had no detectable gammac, four had abnormal gammac expression and no IL-2-induced JAK3 tyrosine phosphorylation, and one had normal gammac expression but no IL-2-induced JAK3 tyrosine phosphorylation, although JAK3 was present. All these patients had mutations identified in their gammac gene. Two patients exhibited normal gammac expression, but JAK3 was not detected by immunoblotting and these patients were confirmed as having JAK3 gene mutations. Thus, these protein-based assays have led to rapid molecular diagnoses in T-B+ SCID that have subsequently been confirmed by genetic analysis.

Nonmyeloablative stem cell transplantation for congenital immunodeficiencies

The optimal approach for stem cell transplantation in children with immunodeficiency has yet to be determined. Conditioning therapy is necessary for reliable engraftment and full immune reconstitution; however, the beneficial effect of cytoreductive conditioning is counterbalanced by increased short- and long-term treatment-related toxicity. Whether bone marrow transplantation with a nonmyeloablative preparative regimen was sufficient for the establishment of donor immune reconstitution, with the resultant correction of disease phenotype, was investigated. Eight patients with severe immunodeficiency states underwent T-cell replete bone marrow transplantation from a human leukocyte antigen-matched unrelated (n = 6) or sibling (n = 2) donor with nonmyeloablative conditioning using a fludarabine-melphalan-anti-lymphocyte globulin-based regimen. All patients had severe organ dysfunction that precluded transplantation with conventional conditioning. All patients were engrafted with predominantly donor hematopoiesis, and the duration of neutropenia was brief. Significant acute graft-versus-host disease (GVHD) did not develop, but one patient had limited chronic GVHD. One patient died of disease recurrence, and 3 have stable, mixed chimerism. At a median follow-up of 1 year, all patients have had good recovery of CD3(+) T-cell numbers, and 6 of 7 evaluable patients have normal phytohemagglutinin stimulation indices. The rate of immune reconstitution is comparable with that of historical controls undergoing standard myeloablative protocols. Two patients with CD40 ligand deficiency now show significant expression, and a patient with adenosine deaminase deficiency has improved deoxy adenosine triphosphate metabolites. In summary, it has been demonstrated that nonmyeloablative stem cell transplantation permits rapid engraftment from both sibling and unrelated donors with minimal toxicity even in the presence of severe organ dysfunction. If long-term immune reconstitution of patients treated with this protocol is demonstrated, it is believed this approach might offer significant advantages compared with standard protocols by combining adequate immune reconstitution with reduced short- and long-term toxicity. (Blood. 2000;96:1239-1246)

Short limbed skeletal dysplasia associated with combined immunodeficiency and congenital subglottic stenosis: a new constellation of features

A newborn male is described with an association of short limbed dwarfism with hip dislocation, combined immunodeficiency characterized by absent B cells and CD4 lymphopaenia and congenital subglottic stenosis. This constellation of abnormalities is distinct from other described skeletal dysplasias associated with immunodeficiency such as ADA deficiency and cartilage hair hypoplasia.

Diagnosis of X-linked lymphoproliferative disease by analysis of SLAM-associated protein expression

X-linked lymphoproliferative disease (XLP) is an inherited immunodeficiency in which affected boys show abnormal responses to Epstein-Barr virus infection. The gene defective in XLP has been identified and designated SH2D1A and encodes a protein termed SLAM-associated protein (SAP). Mutation analysis in individuals with typical XLP presentations and family histories has only detected abnormalities in approximately 60% of patients. Thus, genetic analysis alone cannot confirm a diagnosis of XLP We have developed a SAP expression assay that can be used as a diagnostic indicator of XLP We show that SAP is constitutively expressed in normal individuals, in patients with severe sepsis and in patients with other primary immunodeficiencies. In six XLP patients, four with classical and two with atypical presentations, SAP expression was absent. In the latter two, who were previously assigned as having common variable immunodeficiency (CVID), the diagnosis of XLP was initially made using the protein expression assay. In two further patients in whom no mutation could be detected by genetic analysis, lack of SAP expression strongly suggests that these individuals have XLP. We therefore suggest that XLP should be suspected in certain boys previously diagnosed as having CVID and recommend that patients are investigated both by genetic analysis of SH2D1A and by expression of SAP protein.

Kinase mutant Btk results in atypical X-linked agammaglobulinaemia phenotype

X-linked agammaglobulinaemia (XLA) is a B cell humoral abnormality arising from mutations in the gene encoding Bruton's tyrosine kinase (Btk). The phenotype of XLA can be variable, with some individuals having a less severe immunophenotype, although in most cases this cannot be correlated with the Btk mutation or expression of Btk protein. In this study we describe clinical and immunological heterogeneity within the same pedigree. Analysis of the genetic defect identified a missense mutation in the kinase domain of Btk which, unusually, preserved Btk protein expression but at reduced levels, and also considerably diminished autophosphorylation activity. Structural analysis of the effect of this mutation on the kinase domain suggests that this mutation is not an integral part of the ATP or substrate binding domains but may affect the interaction of the kinase domain with its own kinase domain and other substrates. Together, these data may provide an explanation for the variable XLA phenotype.

Intrinsic defects of B cell function in X-linked severe combined immunodeficiency

The cytokine receptor common gamma chain mutation in X-linked SCID results in a failure of T and NK cell development and an as yet undefined defect of B cells. Using immunoglobulin isotype-specific reverse transcription-PCR we show that although hematopoietic stem cell transplantation restores a diverse repertoire of class-switched B cell clones, on further analysis these are almost all of donor origin. This suggests that host B cells, which predominate after unconditioned transplantation, are still defective even in the presence of normal T cells. These studies imply that effective humoral reconstitution can only be achieved by the engraftment of normal donor B cells.

Immunogenetics: changing the face of immunodeficiency

Tables 1 and 2 highlight the enormous advances that have been made in the definition of the molecular defects underlying primary immunodeficiencies in the past decade. The identification of SAP as the gene defective in XLP now completes the molecular bases of all the recognised X linked syndromes. Of the autosomally inherited syndromes, only the genes for DiGeorge syndrome, hyper-IgE, and perhaps most importantly, common variable immunodeficiency remain to be elucidated. The major clinical benefits of this information have primarily been in offering more accurate and rapid molecular diagnoses. The ability to make a molecular diagnosis also increases the options for earlier definitive treatments such as bone marrow transplantation and somatic gene therapy. Finally, as illustrated by the studies on the functions of WASP and the gamma c/JAK-3 pathway, identification of the gene defect is the first step to understanding the molecular pathogenesis of the immunological abnormalities.

Absence of expression of the Wiskott-Aldrich syndrome protein in peripheral blood cells of Wiskott-Aldrich syndrome patients

Wiskott-Aldrich syndrome (WAS) is an X-linked primary immunodeficiency that is usually associated with thrombocytopenia and eczema. The very variable phenotype of WAS results from defects in the WAS protein (WASP), the function of which is not well understood. In many cases causative mutations have now been identified in the WAS gene. Attempts have been made to correlate the nature of the mutations with the severity of the disease. In this study we investigated mutations in 13 patients with WAS and analyzed the expression of WASP in patient blood samples by immunoblot analysis. We found that despite extensive variation in the nature of the mutations in patients with severe WAS symptoms, none express the protein. However, in 1 patient with a mild clinical phenotype WASP expression was detected. Such an analysis could be used as an initial screening procedure for the diagnosis of WAS prior to genotypic analysis.

Bruton's tyrosine kinase expression and activity in X-linked agammaglobulinaemia (XLA): the use of protein analysis as a diagnostic indicator of XLA

Mutations in the Bruton's tyrosine kinase (BTK) gene result in XLA. Despite the large numbers of BTK mutations reported, no correlation can be made between the clinical phenotype and the gene defects. Analysis of Btk protein expression and activity in individuals with XLA was performed to characterize the relationship between a particular mutation, the resultant Btk protein and the clinical phenotype. In most patients studied, including those with atypical phenotypes, there was complete absence of protein expression and activity. Furthermore, in two undiagnosed individuals with a clinical phenotype suggestive of XLA, lack of protein expression was used to confirm an abnormality in Btk. These results underline the importance of protein analysis prior to speculating on protein structure and function based on the gene mutation. Lack of Btk expression in atypical phenotypes suggests that there is redundancy in B lymphocyte signalling such that alternative signalling molecules, or mechanisms, can compensate for the lack of Btk. We also suggest that analysis of Btk expression can be used as an indicator of XLA. These rapid assays may be used to screen a wider spectrum of individuals with humoral immunodeficiency in order to characterize fully the extent of Btk deficiency.

Immunodeficiency syndromes and recurrent infection

Individuals with increased susceptibility to infection may have an acquired or inherited abnormality of the immune system. In the second of two reviews we aim to outline some of the most important immunodeficiency syndromes that may be associated with a clinical presentation of frequent infection. Awareness of such syndromes may aid pattern recognition and the appropriate investigation of patients with recurrent infection.

Investigation of the immune system in recurrent infection

Individuals with increased susceptibility to infection may have an acquired or inherited abnormality of the immune system. The complexities of human immunity are such that to investigate the cause may appear a daunting prospect. However, in this review we aim to outline the basic components of the immune system and to provide a logical and stepwise approach to the investigation of immunodeficient individuals.

Expression of Bruton's tyrosine kinase protein within the B cell lineage

Defects in the gene encoding Bruton's tyrosine kinase (Btk), normally expressed in B cells, cause X-linked agammaglobulinemia (XLA). The phenotype of XLA is characterized by a lack of circulating B cells and immunoglobulin. It has been suggested that B cell maturation from the pre-B cell stage to more mature stages is dependent on the appropriate expression of this gene. The Btk mRNA is expressed in B cells and myeloid cells, but protein expression in relation to B cell maturation has not been determined. Moreover, expression of the Btk protein has so far only been investigated in human Epstein-Barr virus-transformed B cell lines, and in murine splenocytes and B cell lines. We have developed an antiserum which recognizes the human Btk protein and shown that normal human tonsillar B cells, peripheral blood monocytes and myeloid cells express the protein, whereas tonsil-derived T cells do not. We also show that the protein is present in early and mature human B cell lines, but is absent in terminally differentiated plasma cell lines. Furthermore, expression is reduced or absent in three B lineage cell lines derived from two patients with defined genetic mutations in Btk and suffering from XLA.